For patients with healthy lungs, no significant phase shift exists between the force response of the respiratory muscles and the resulting respiratory flow. For this reason, a good respiratory phase identification can be derived from the zero crossovers of the mouth pressure or respiratory flow as they are required as a trigger criterium for the phase reversal during assisted ventilating forms.
U.S. Pat. No. 3,834,382 discloses a ventilating apparatus wherein the zero crossover of the respiratory flow curve is used as a trigger criterium. This method however is deficient when the time constant of the lung with reference to the duration of the respiratory phases increases as is the case for chronically obstructive patients (COPD) at increased breathing frequency. A phase shift occurs between the force response of the respiratory muscles and the resulting respiratory flow. A discoordination between the activity of the respiratory muscles and the flow direction in the respiratory tract is observed and thereby the externally recognizable respiratory phases.
An assisted ventilating method which undertakes the reversal of inspiration to expiration or from expiration to inspiration pursuant to methods of respiratory phase identification used today can, in such patients, contribute to an unwanted increase of the isometric respiratory work. The patient senses the initiation of the inspiration assistance as being too late; on the other hand, the patient already initiates an expiration even though the ventilating apparatus still continues to supply volume to the lung.
U.S. Pat. No. 4,050,458 discloses a ventilating apparatus wherein the time trace of the mouth pressure of the patient is measured and, by forming the second time differential, a kink point in the end phase of the expiratory pressure trace is determined. This kink point acts as a trigger criterium for the inspiration phase which follows. It is a disadvantage of this apparatus that a reliably evaluatable pressure signal is only obtained when the ventilating apparatus has an unwanted high flow resistance. For conventional flow resistances, the pressure signal is so small that it merges with noise. Furthermore, it is a disadvantage that no trigger criterium can be obtained for the reversal from inspiration to expiration.